.\"	$OpenBSD: physio.9,v 1.7 2010/09/22 01:18:57 matthew Exp $
.\"	$NetBSD: physio.9,v 1.5 1999/03/16 00:40:47 garbled Exp $
.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Paul Kranenburg.
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.Dd $Mdocdate: September 22 2010 $
.Dt PHYSIO 9
.Os
.Sh NAME
.Nm physio
.Nd initiate I/O on raw devices
.Sh SYNOPSIS
.Ft int
.Fo "physio"
.Fa "void (*strategy)(struct buf *)"
.Fa "dev_t dev"
.Fa "int flags"
.Fa "void (*minphys)(struct buf *)"
.Fa "struct uio *uio"
.Fc
.Sh DESCRIPTION
.Fn physio
is a helper function typically called from character device read and write
routines to start I/O on a user process buffer.
It calls back on the provided
.Fa strategy
routine one or more times to complete the transfer described by
.Fa uio .
The maximum amount of data to transfer with each call to
.Fa strategy
is determined by the
.Fa minphys
routine.
Since
.Fa uio
normally describes user space addresses,
.Fn physio
needs to lock the appropriate data area into memory before each transaction
with
.Fa strategy
(see the
.Fn uvm_vslock
and
.Fn uvm_vsunlock
functions in
.Xr uvm 9 ) .
.Fn physio
always awaits the completion of the entire requested transfer before
returning, unless an error condition is detected earlier.
.Pp
In all cases, a temporary buffer is allocated from a system pool.
This buffer will have the
.Dv B_BUSY ,
.Dv B_PHYS ,
and
.Dv B_RAW
flags set when passed to the strategy routine.
.Pp
A break-down of the arguments follows:
.Bl -tag -width indent
.It Fa strategy
The device strategy routine to call for each chunk of data to initiate
device I/O.
.It Fa dev
The device number identifying the device to interact with.
.It Fa flags
Direction of transfer; the only valid settings are
.Dv B_READ
or
.Dv B_WRITE .
.It Fa minphys
A device specific routine called to determine the maximum transfer size
that the device's strategy routine can handle.
.It Fa uio
The description of the entire transfer as requested by the user process.
Currently, the results of passing a
.Fa uio
structure with the
.Sq uio_segflg
set to anything other than
.Dv UIO_USERSPACE ,
are undefined.
.El
.Sh RETURN VALUES
If successful,
.Fn physio
returns 0.
.Er EFAULT
is returned if the address range described by
.Fa uio
is not accessible by the requesting process.
.Fn physio
will return any error resulting from calls to the device strategy routine,
by examining the
.Dv B_ERROR
buffer flag and the
.Sq b_error
field.
Note that the actual transfer size may be less than requested by
.Fa uio
if the device signals an
.Dq end of file
condition.
.Sh SEE ALSO
.Xr read 2 ,
.Xr write 2
